Updated 10 September 2010

Concept

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The Direct Shear Test is used for determination of the consolidated drained (or undrained) shear strength of soils. The test is performed by deforming a specimen at a controlled rate on or near a single shear plane.

Description and test procedure

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The direct shear test is a laboratory testing methods used to determine the shear strength parameters of soil. The test can be carried out at different moisture contents; however, it is common to saturated the sample before running the test.  To achieve reliable results, the test is often carried out on three or four samples of undisturbed soil. The soil sample is placed in a cubic shear box composed of a upper and lower box. The limit between the two parts of the box is approximately at the mid height of the sample.

The sample is subjected to a controlled normal stress and the upper part of the sample is pulled laterally at a controlled strain rate or until the sample fails. The applied lateral load and the induced strain are recorded at given internals. These measurements are then used to plot the stress-strain curve of the sample during the loading for the given normal stress.  

Results of different tests for the same soil are presented in a chart with peak stress on horizontal axis and normal (confining) stress on the vertical axis. A linear curve fitting is often made on the test result points. The intercept of this line wit the vertical axis gives the cohesion and its slope gives the peak friction angle.